The present invention relates to an apparatus for and a method of supervising the energy level of a battery, and in particular to such an apparatus and method for use with a device for detecting the occurrence of a predetermined condition.
Battery powered equipment is generally unreliable to the extent that the energy level of the battery ultimately will decrease below a level sufficient to properly operate the equipment. Where the equipment is, for example, a fire detector, allowing the energy level of the battery to decrease below a level sufficient to operate the detector not only is undesirable, but is intolerable, since such would result in the detector becoming inoperable and failing to generate an alarm upon the occurrence of combustion.
Merely instructing a user of a fire detector to replace the battery after a predetermined peiod of time is unsatisfactory, since it is likely that the instructions will either be forgotten or unheeded, ultimately resulting in failure of the detector. Further, due to variations between individual batteries which cause the lengths of their useful lives to vary considerably one from the other, it is possible that a battery may fail before the end of its expected useful life. Consequently, battery powered devices such as fire detectors should be provided with some means for warning users of impending battery failure.
A decrease in the energy level of a battery generally is evidenced both by a decreased battery output voltage and an increased battery internal resistance. A decrease in battery output voltage limits the power which the battery is capable of providing, and an increase in battery internal resistance limits the output current and, therefore, the power output of the battery.
One conventional technique for monitoring the energy level of a battery contemplates comparing the output voltage of the battery with a predetermined and lower voltage. The predetermined voltage is that level of battery voltage at which failure of the battery is expected to be impending, and upon the battery voltage reaching the predetermined level as the energy level in the battery decreases, an alarm is generated to indicate the need for replacement of the battery. A disadvantage of this technique is that prior to the battery output voltage decreasing to the predetermined value it is possible, due to variations in the battery, for the internal battery resistance to increase to a value at which the battery is incapable of properly powering the device in all of its modes. In the case of a fire detector the warning generated in response to combustion often comprises the sounding of a horn. Horns usually require relatively large currents for their operation, and it is possible that the internal resistance of the battery may, prior to its output voltage decreasing to the level sufficient to generate the warning, increase to a level whereat the battery will not be capable of operating the horn should combustion occur.
Another technique for monitoring the energy level of a battery is disclosed in U.S. Pat. No. 3,899,732 to Paul A. Staby, wherein the output voltage and the internal impedance of the battery is monitored and used as an indication of the energy level of the battery. In this case, the battery is periodically loaded, a first signal is generated which has a magnitude equal to a selected proportion less than 100% of the output voltage of the battery prior to loading, and a second signal is generated having a magnitude less than the first signal and equal to the output voltage of the battery when loaded less a predetermined voltage drop. The first and the second signals are compared, and a third signal is generated upon a predetermined difference therebetween. A disadvantage of the Staby circuit is that the first and the second signals follow naturally occurring decreases in battery voltage, with the second signal directly following the same and the first signal following only a proportion of the same, with the result that as the battery voltage decreases the sensitivity of the energy level detection is increased. This undesirably may result in premature generation of a signal of impending battery failure.
Accordingly, it is an object of the present invention to provide improved apparatus for and methods of supervising the energy level of a battery and providing a warning of impending battery failure.
Another object of the invention is to provide apparatus for and methods of monitoring the energy level of a battery and determining if the same exceeds a selected level.
Yet another object of the invention is to provide such apparatus and methods for providing an indication of impending battery failure only upon the internal impedance of the battery exceeding a predetermined value.
A further object of the invention is to provide such apparatus and methods, wherein the battery periodically is loaded and a warning of impending battery failure is generated only upon detection of a predetermined difference between the output voltages of the battery prior to loading and during loading.
A still further object of the invention is to provide such apparatus and methods, wherein the sensitivity of the supervising apparatus is substantially independent of naturally occurring decreases in battery voltage prior to loading.